An engine control module of an internal combustion engine controls the mixture of fuel and air supplied to combustion chambers of the engine. After the spark plug ignites the air/fuel mixture, combustion takes place and later the combustion gases exit the combustion chambers through exhaust valves. The combustion gases are directed by an exhaust manifold to a catalytic converter or other exhaust after treatment systems.
During certain engine operating periods, combustion gases that enter the exhaust manifold are not completely burned. The combustion gases will continue to burn in the exhaust manifold if a sufficient amount of oxygen is available. Secondary air injection systems are used to inject additional air into the exhaust flow to allow combustion to continue, which improves the performance of exhaust after treatment systems and reduces emissions.
In addition, a turbocharger can be added to an engine to provide enhanced performance and reduced emissions. Twin scroll technology is often used to further enhance the performance of a turbocharged engine; in particular inline four or six cylinder engines as well as those having “V” or “flat” architectures. In engines featuring twin scroll or twin turbo technology, the exhaust manifold of the engine is designed to group the cylinders so the events of the cylinders in each group are separated to minimize cylinder-to-cylinder exhaust flow interference. For example, cylinder groups may be formed to provide sequences of high pulse energy to drive the turbine wheel, as each group experiences combustion, thereby maintaining a strength or magnitude of exhaust pulses. Thus, a first group of cylinders that is substantially out of phase (substantially not firing) in relation to a second firing group of cylinders does not interfere with or degrade an exhaust pulse caused by igniting the second group of cylinders. Accordingly, twin scroll turbocharger systems have increased forces imparted on the turbine wheel to improve turbine performance. However, secondary air injection systems may allow cross talk or degradation between cylinder groups by allowing communication between the exhaust flow of cylinder groups, thereby reducing energy provided by exhaust pressure pulses.